1. Field of the Invention:
The present invention relates to a boiler furnace combustion system, and more particularly to improvements in an electric utility or industrial boiler furnace combustion system.
2. Description of the Prior Art:
At first, one example of a boiler furnace in the prior art will be explained with reference to FIGS. 5 to 7. Among these figures, FIG. 5 is a vertical cross-sectional view; FIG. 6 is a horizontal cross-sectional view taken along line VI--VI in FIG. 5; and FIG. 7 is another horizontal cross-sectional view taken along line VII--VII in FIG. 5.
In these figures, reference numeral 01 designates a boiler furnace main body, numeral 02 designates main burner wind boxes, numeral 03 designates main burner air nozzles, numeral 04 designates main burner fuel injection nozzles, numeral 05 designates air ducts for introducing air to the main burners, numeral 06 designates fuel feed pipes, numeral 07 designates additional air ducts, numeral 09 designates flames, numeral 10 designates air for the main burners, numeral 11 designates fuel such as pulverized coal, petroleum, gaseous fuel or the like, numeral 12 designates additional air, numeral 13 designates unburnt combustion gas, numeral 14 designates combustion exhaust gas, numeral 15 designates wind boxes, numeral 16 designates air nozzles, and numeral 20 designates imaginary cylindrical surfaces.
At lower corner portions of a square-barrel-shaped boiler furnace main body 01 having a nearly vertical axis are respectively provided main burner wind boxes 02, and at upper corner portions of the same main body are respectively provided wind boxes 15 for additional air (hereinafter abbreviated as AA). Within each main burner wind box 02 there is provided main burner fuel injection nozzles 04 and main burner air nozzles 03 extending nearly horizontally.
Fuel 11 is fed from a fuel feed installation (not shown) to the main burner fuel injection nozzles 04 through the fuel feed pipes 06 and is injected into the boiler furnace 01. On the other hand, main burner air 10 is fed from a ventilating installation (not shown) through the main burner air ducts 05 to the main burner wind boxes 02, and is blown into the boiler furnace 01 through the main burner air nozzles 03.
The injection of the fuel 11 and of the main burner air 10 is effected in a direction tangential to an imaginary cylindrical surface 20 which is located at the central portion of the boiler furnace 01. The fuel 11 injected into the boiler furnace 01 along the tangential direction is ignited by an ignition source (not shown) to form flames 09, and as the fuel diffuses and mixes with the main burner air 10 injected in the tangential direction through the main burner air nozzles 03, combustion is continued.
The main burner air 10 is fed at a rate lower than an air feed rate that is theoretically necessary for combusting the fuel 11 injected into the boiler furnace 01. Therefore, the interior portion of the boiler furnace 01 below the AA blowing portion is held under a reducing atmosphere. Accordingly, the combustion of the fuel 11 produces unburnt combustion gas 13 containing unburnt fuel at the portion below the AA blowing portion.
The AA 12 is fed from a ventilating installation (not shown) which also feeds the main burner air 10, or from a separately disposed ventilating installation (not shown) through the AA ducts 07. The AA 12 is blown into the boiler furnace 01 in a tangential manner, like the main burner air 10, through the AA air nozzles 16 disposed nearly horizontally in AA wind boxes 15. Normally, the injection of the AA 12 is effected in the same tangential direction as the main burner air 10 with respect to the imaginary cylindrical surface 20. The flow rate of the AA 12 is such that a sufficient amount of oxygen, i.e. an amount necessary for perfectly burning unburnt fuel in the unburnt combustion gas 13, is fed into the boiler furnace 01.
The AA 12 blown into the boiler furnace 01 is mixed with the unburnt combustion gas 13 by diffusion, thus causing the unburnt fuel in the unburnt combustion gas 13 to burn perfectly, and is exhausted to the outside of the boiler furnace 01 as combustion exhaust gas 14.
In such a boiler furnace in the prior art, the combustion of the fuel 14 injected through the main burner fuel injection nozzles 04 produces some unburnt combustion gas 13 due to the fact that the flow rate of the main burner air 10 is less than the theoretical air flow rate. And, the interior portion of the boiler furnace below the AA blowing portion is under a reducing atmosphere. Consequently, in that portion below the AA blowing portion, the amount of nitrogen oxides (hereinafter represented by NO.sub.x) produced by the combustion of the fuel 11 is small, and instead intermediate products such as ammonia (NH.sub.3), cianic acid (HCN) and the like are produced.
Subsequently, in the AA blowing portion, it is desired to completely combust unburnt components of the unburnt combustion gas 13 by injecting AA 12 through the AA blowing nozzles 16. At that time since the intermediate products such as NH.sub.3, HCN and the like tend to be oxidized and transformed into NO.sub.x, the injection of AA 12 is carried out in a relatively low-temperature (about 1000.degree.-1200.degree. C.) atmosphere within the boiler furnace 01 for the purpose of suppressing the transformation rate of the intermediate products into NO.sub.x.
And because the flow rate of the main burner air 10 is less than the theoretical air flow rate necessary for the air to completely combust with the fuel 11, the unburnt combustion gas 13 rises while swirling. As the unburnt combustion gas 13 rises, the outer diameter of the swirling flow of the unburnt combustion gas 13 gradually becomes large, and in the proximity of the AA blowing portion, the amount of unburnt combustion gas 13 flowing along the wall of the boiler furnace 01 increases.
The blowing momentum of the AA 12 is about 1/5 to 1/3 that of the blowing momentum of the main burner air 10, provided that the blowing velocities are equal to each other. The AA 12 blowing through the AA blowing nozzles 16 at the respective corner portions both diffuses and mixes with the main flow portion of the unburnt combustion gas 13, and penetrates through the main flow portion and flows towards the central portion of the boiler furnace 01. The momentum of the AA 12 flowing towards the central portion of the boiler furnace 01 is attenuated due to the facts that the AA 12 has penetrated through the main flow portion of the unburnt combustion gas and that the distance from the AA blowing nozzle 16 to the central portion of the boiler furnace 01 is long. Hence, the AA 12 does not diffuse or mix with the unburnt combustion gas 13 in the proximity of the central portion of the boiler furnace 01. Accordingly, the AA 12 rises without contributing to the completion of the combustion of the unburnt combustion gas, and it is exhausted from the outlet of the boiler furnace 01.
Therefore, in order to complete the combustion of the unburnt components of the unburnt combustion gas 13 within the boiler furnace 01 in the prior art, countermeasures such as (1) increasing a total combustion air flow rate (a flow rate of main burner air 10 + a flow rate of AA 12), (2) lengthening the time in which it takes combustion gas from the AA blowing portion to flow to the outlet of the boiler furnace 01, (3) weakening the reducing atmosphere under the AA blowing portion by increasing a flow rate of the main burner air 10, or the like are necessary. However, countermeasures (1) and (3) are disadvantageous in view of the production of NO.sub.x, and the countermeasure (2) is disadvantageous in view of cost.
As described above, the boiler furnace combustion system in the prior art presents problems in connection with the diffusion and mixing of the AA 12 and the unburnt combustion gas 13. Therefore, there is a problem to be resolved in that if one intends to decrease NO.sub.x production, the amount of unburnt fuel is increased, while if one intends to decrease the amount of unburnt fuel remaining, NO.sub.x reduction is not sufficient.